As is known, a user authentication by a fingerprint recognition is not only convenient to use but also excellent in terms of a security and cost-effectiveness and is broadly commercialized now. A device for obtaining a fingerprint input image is generally classified into an optical device using optical sensors and a semiconductor type device using semiconductor touch sensors. Of these devices, an optical sensor has a strong durability (a scratch resistance, chemical resistance, abrasion resistance, and shock resistance) and exhibits a high recognition rate due to the visibility of an image (an image resolution), and thus the optical device is more preferred.
An optical fingerprint input device using the optical sensor has an advantage in light of the visibility of the image and recognition accuracy, but it shows a shortcoming that the quality of fingerprint images fluctuates with the change in a primary intensity of radiation due to an inflow of external light, which is one of the main drawbacks.
However, in general, because part of a finger does not entirely cover a prism area in the optical fingerprint input device, the inflow of external light is inevitable. While the design of the optical system is taken into consideration to avoid the inflow of external light, when there is an inflow of the external light having a level of 10000˜100000 Lux as strong as the light of the sun, which results in obtaining a fingerprint image created by an internal illumination, as well as a fingerprint image created by the external light. For these reasons, most of the optical fingerprint input devices have been developed for indoor use, and a number of constraints are required, or the indoor use is impossible if the fingerprint input devices are used in outdoor.
An Adaptive Gain Control (AGC) may be employed in order to avoid the demerit caused by the inflow of strong external light. The AGC performs a process looking for an optimal fingerprint image while reducing the exposure value of a camera.
However, it is difficult to predict the intensity of external light to flow in, and it takes much more time to perform the AGC. Further, because the exposure caused by arbitrary external light is adjusted, there may occur a fluctuation, the so-called Hunting phenomenon wherein the amount of the control does not stop to vibrate above and below a predefined value. Owing to this, it is extremely difficult to use the optical fingerprint input device in outdoor.
Further, for a scattering-type optical system, because the scattering is done in an overall area of the finger due to strong external light, it appears that an entire region of the fingerprint is saturated bright. For an absorption-type optical system, because an intensity of external light transmitted through a finger (external light received and scattered inside a hand) becomes brighter than that of the total reflection of a prism, ridges and background may be viewed in a reverse image. In the fingerprint input device, a significant problem owing to the external light is that it is difficult to determine whether or not an input image is a fingerprint image, it takes a lot of time to perform the AGC, or it is impossible for the AGC to conduct even though the AGC has been conducted to make an image quality recognizable.